Prior to the present invention, it has been an important objective in the polyolefin industry to develop polyolefins which are resistant to thermal degradation and the degradation caused by exposure to light and ultraviolet rays. Various kinds of hindered amines--"hindered amine light stabilizers" ("HALS")--have been widely used for these purposes. They have been found to be effective in stabilizing against light when blended into the polyolefin, but migration and leaching out by the relatively mobile hindered amines can result in reduced effects against light. On the other hand, oligomeric hindered amine light stabilizers, such as Chimassorb 944 and the like, generally do not migrate or leach out. But these stabilizers have been found to be less effective in stabilizing against light, compared with relatively mobile hindered amine light stabilizers. Furthermore, since currently commercially available HALS can not adequately protect against heat, thermal stabilizers have to be added. Many commonly used HALS, however, show negative synergy with the most widely used class of thermal stabilizers, phenolic antioxidants. It would be highly desirable to have a light stabilizer which does not show negative synergy with phenolic stabilizers at high temperatures. Thus the art is constantly looking for improved methods of protecting polyolefins and polyolefin products from degradation caused by light and heat.
So far as we are aware, a practical copolymer of propylene or other alpha-olefin has not been made with a comonomer containing an effective hindered amine light stabilizer as a structural unit, hereinafter sometimes called HALS comonomer. Such a copolymer would be highly desirable not only for medical applications as mentioned above, but for other applications of polypropylene which require resistance to degradation from heat and visible light. Some types of medical products would benefit from protection against degradation caused by repeated autoclaving. Such copolymers containing relatively low concentrations of HALS comonomers could be used by themselves and would be expected to withstand the conditions of injection molding; similar copolymers, perhaps containing higher concentrations of HALS comonomers, could be blended with less expensive propylene homopolymers, and would not migrate as the unattached hindered amines have been known to do.
In U.S. Pat. No. 3,975,357, Murayama et. al. describe a large class of N-substituted 2,2,6,6-tetramethylpiperidines which may be used as heat and light stabilizers. The described class includes compounds of the formula ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 can be methyl and R.sup.5 can be an alkenyl group of 3-10 carbon atoms. No attempt was made to copolymerize them, however; they were simply blended with a host polymer to stabilize it, in a manner no different than many other unpolymerizable stabilizer additives.
In U.S. Pat. Nos. 4,857,595 and 4,975,489, Kazmierzak and MacLeay describe "polymer-bound" hindered amine light stabilizers. The polymers are made to contain hindered amines by reacting hydrazido substituted hindered amines with, for example, maleic anhydride modified polypropylene. Similar reactions are described by MacLeay and Myers in U.S. Pat. Nos. 4,683,999 and 4,868,246. One must first make the maleic anhydride-modified resin, however, an expensive separate step which tends itself to degrade polypropylene or other thermoplastics. While the HALS-containing moiety is polymer-bound, it is not incorporated in the polymer backbone by original copolymerization. See also German patent 4,436,355 to Bartus et al (Chemie Linz) and U.S. Pat. No. 4,473,657 to Rekers et al (Deering Milliken) which use HALS maleimides to combine in a reactor/extruder with preformed polymers, making grafted polymer-bound hindered amine light stabilizers. These materials also do not employ the HALS component as an original comonomer with the propylene or other original .alpha.-olefin monomer.
Other types of grafting attempts are described in Japanese JP Kokai 25330/94. Grafting onto functional groups such as aldehyde groups has definite disadvantages, such as low yields associated with undesirable unreacted monomer, and degradation of the physical properties of the base polymer.
Sevini and Noristi, in U.S. Pat. No. 4,960,593, propose the intimate incorporation of free hindered amines in polypropylene by including them in the polymerization reaction mixture at the time of initiation of the reaction, or sometime during the course of the reaction, or afterwards but before the catalyst is quenched. As stated at the end of their Example 10, the timing of the addition is not critical; the inventors perceive no difference so long as the HALS compound is introduced before the catalyst is quenched. Thus, in spite of the fact that their vastly comprehensive general formulas include compounds employed in the present invention, it is clear that the patentees do not intend to copolymerize them; in fact, all but a few of their HALS compounds could not possibly copolymerize, being without polymerizable functional groups. The HALS compounds are simply incorporated into the mixture in perhaps a more intimate manner than has been the case for more conventional polyolefins in which the HALS compound is blended after the catalyst is quenched. Neither this patent nor any other of which we are aware discloses a copolymer of propylene with a copolymerizable hindered tertiary amine compound of the formula (I) below.
We are aware of the work of Wilen, Markku, Auer and Nasman, J. Polymer Sci., Part A-Polymer Chemistry, v 30, 1163-1170 (1992), who copolymerized certain hindered secondary amines with propylene. The polymerizable group was attached to the 4-position of the piperidine group, such as in 4-(hex-5-enyl)-2,2,6,6-tetramethylpiperidine. Because this reactant is a secondary amine, i.e. the 1-position is &gt;NH, polymerization catalytic activity is drastically reduced and can only be restored by using excessively large amounts of triethyl aluminum (TEA); multiple reactions are needed to obtain significant yields. We use hindered amines, by contrast, in which the polymerizable group is at the 1-position of the piperidine group, making the amine a tertiary amine.
Langer and Haynes, in U.S. Pat. No. 3,755,279, use as comonomers 1-alkenylpiperidines which are hindered only enough to avoid destruction of the polymerization catalyst, yet the amine sites can be used for dyeing. The 2- and 6-positions of the piperidine are occupied by only one alkyl group each. These compounds are not highly hindered around the nitrogen atom and their efficacy as a light stabilizers is expected to be much less than our comonomers. The present application is concerned particularly with 1-alkenyl piperidines wherein the 2- and 6- positions are fully substituted to develop the light and heat stabilization.